This application claims priority of German Patent Application No. 102005013855.1 filed Mar. 24, 2005, the disclosure of which is herein incorporated by reference in its entirety.
The present invention relates to a column for mass exchange between a liquid and a gas. The column has at least one pair of adjacent exchange plates, formed by an upper exchange plate and a lower exchange plate; a drain shaft for passing liquid; and an inlet cup located underneath the bottom end of the drain shaft and which has a bottom that is recessed relative to the surface of the lower exchange plate.
Plate columns are often used for mass exchange processes, especially as washing columns or as rectification columns for gas separation. Their basic structure is described in, for example, Winnacker-Küchler, Chemische Technologie [Chemical Technology], Vol. 7, 3rd Edition (1975), Section 3.35, pp. 196-200.
Different types of exchange plates are known, such as, for example, perforated plates, bubble-cap plates, or valve plates. The present invention can be applied to all types of exchange plates.
In so-called crossflow plate columns, the liquid runs transversely to the rising gas over an upper exchange plate and then flows by way of one or more drain shafts to the underlying lower exchange plate. On the exchange plates, the liquid through which the gas has flowed forms an effervescent layer in which gas and liquid enter into intense direct mass exchange.
The “surface” of a plate is defined as the plane that stops the liquid, which flows crosswise over the plate and is in mass exchange with the rising gas, from flowing off directly to the bottom and thus defines the lower interface of the effervescent layer. In a perforated plate, the surface of the plate is formed, for example, by the surface of a perforated sheet from which the plate is conventionally produced.
The spatial terms “upper”, “top”, “lower”, “bottom”, “vertical,” and the like, refer to the orientation of the column during operation.
Since the pressure in the plate column rises from top to bottom, the pressure is higher in the vapor space of the lower plate than on the upper plate. The liquid therefore banks up in the drain shaft until the weight of the banked liquid is sufficient to convey the liquid into the vapor space of the lower plate. This liquid backup level determines the required plate interval and thus the column height in many cases.
The backup level of the liquid in the drain shaft is determined by several factors.
Generally, the pressure drop of the plate yields the greatest portion for the backup level. The rising gas undergoes this pressure drop when flowing from the lower exchange plate through the exchange elements of the upper exchange plate and when overcoming the hydrostatic height of the effervescent layer of the upper exchange plate.
Generally, the friction term yields a smaller portion for the backup level in the drain shaft and is necessary to force the liquid through an outlet gap or through several outlet openings onto the lower exchange plate. In exchange plates having an inlet weir on the lower plate, which ensures a static liquid seal of the shaft in any case of operation, a significant portion of the entire backup level may be produced.
This additional portion of the backup level is avoided in many modern plates by abandoning a static liquid seal and not using an inlet weir, or by arranging the outlet openings of the shaft above the effervescent layer. The shaft seal is then produced dynamically by the pressure drop that arises on the outlet openings.
It is known from Mayfield et al., Journal of Industrial and Engineering Chemistry (1952), 44, 2238-49 (FIG. 13) to place a recessed drain cup underneath the drain shaft and thus to produce a static liquid seal without an inlet weir. However, this incurs a relatively high cost in the production of the column.
Therefore, an object of the present invention is to devise a plate column with a recessed inlet cup that can be produced economically and at the same time functions advantageously in process engineering terms.
This object is achieved in that the recessed inlet cup is made such that it extends over only part of the cross-section of the bottom end of the drain shaft.
Within the framework of the present invention, it has been ascertained that a relatively small passage area is sufficient for runout of the liquid from the drain shaft onto the lower exchange plate. Therefore, in contrast to the teaching of the prior art, it is not necessary to dip the entire drain shaft into the recessed cup. Rather, one recess in the plate in the area of the passage gap is adequate. This recess can be made easily and economically by a folded sheet metal cup. The latter can be joined to the exchange plate by means of simple connecting elements, for example, rivets. Thus, special effort for sealing the inlet cup relative to the column wall becomes unnecessary. Nevertheless, the favorable effect of the recessed inlet cup can be used for the backup level in the drain shaft and thus for the overall height of the entire column.
The recessed inlet cup extends, for example, over 20% to 60% of the lower cross-sectional surface of the drain shaft, preferably over 25% to 40%.
Preferably, the drain shaft ends above the surface of the lower exchange plate. In this way, the inlet cup by itself no longer forms a static liquid seal. This can be compensated for by conventional measures for producing a dynamic liquid seal by correspondingly configured outlet openings of the drain shaft.
It is more advantageous, however, if, instead, an inlet weir is located on the edge of the inlet cup facing the lower exchange plate and produces a static liquid seal.
Within the framework of the present invention, this inlet weir can be designed very small. In particular, it is advantageous if the inlet weir's height is less than, equal to, or not much higher than the height of the drain weir of the lower exchange plate. “Not much higher” means exceeding the height of the drain weir by 50% or less, preferably by 20% or less, at most preferably by 10% or less, or by 5% or less. The inlet weir is thus lower than the height of the effervescent layer on the lower exchange plate in normal operation. A static liquid seal is produced by interaction of the recessed inlet cup with a low inlet weir without contributing to a significant increase of the liquid backup level. In contrast to dynamic liquid seals, the column is nevertheless suitable for a wide range of loads. The height of the inlet weir may overlap the lower edge of the drain shaft wall by 0 to 20 mm, preferably by 0 to 10 mm.
Here, “normal operation” is defined as operation of the column with a gas and liquid load that lie within the load range for which the column is designed.
With the aid of a recessed inlet cup, a relatively small plate spacing is possible. For high gas loads with a highly expanding effervescent layer, the recessed inlet cup of the upper exchange plate can prevent drainage of the two-phase layer from the lower exchange plate. This disadvantage can be avoided or at least mitigated by the drain shaft's tapering downward, for example, by the wall of the drain shaft facing the lower plate being located obliquely to the vertical. The cross-sectional area of the bottom tapered end of the drain shaft is, for example, 10% to 80% of the cross-sectional area of the top end of the drain shaft, preferably 30% to 50%.
Of course, the present invention can be applied analogously to multiflood plates in which there is more than one drain shaft between a pair of adjacent exchange plates.
The present invention and other details of the invention are explained in more detail below using the embodiments shown in the drawings.